Is he eating normally?
To make sure he isn't dangerously skinny, feel along his spine. If the muscle is parallel, he's healthy. If it is sunken, that means he has no fat stores and is burning muscle to survive. This is the too skinny that requires rescue diet.
He has long enough fur, you should check his poops; if they are smaller than usual, or if you find 'pearl necklaces' (poops joined together by bits of fur) you might have a mild case of wool block. Feed some pineapple, to dissolve the fur buildup in the gut, and lots of fiber to push the blockage through.
If he's eating normally, and you want him to gain weight, give him more treats. Sunflower is great, since they eat the shells as well, which is good fiber.